Planting Onions in Containers

This year I decided to experiment with a few new crops, and one of them is the humble, versatile, and delicious onion! After much research here is what I decided on for this season. Will it work? We'll find out together!

Step #1: Select an onion type that is appropriate to your region

This is key, because onions form bulbs based on the number of hours of daylight they receive. Your latitude determines the best type of onions for your region. There are many great resources for further reading on this topic, but it basically boils down to this:

Short Day Onions are best in the south. If you plant a short day onion in the north it will receive the required number of hours of daylight to trigger bulb formation early in the season before it has formed a strong root system and enough greens to support the bulb. As a result, it will grow greens but you will only get a tiny bulb. The unused package of Granex (Vidalia) seeds I have in my seed basket is a great example of this type (do your research before you buy!).

Long Day Onions are best in the north. These require very long days to form a bulb, so they will set their roots and grow their greens in the north in the cool of spring, then begin to set nice bulbs during the summer when daylight hours are much longer. If you plant a long day onion in the south it will never get the required number of hours of daylight to bulb, so all you will get is greens.

Intermediate Day (Day Neutral) Onions can be grown in both regions. They should work well in most growing regions, aside from the extremes.

After I read up on daylight hours I selected two day neutral types: Candy onions and a red onion.

Red onions planted in early April

Step #2: Seeds, sets, or plants?

This is another consideration. You can grow your onions from seeds, sets (small onion bulbs), or small plants. Seeds are cheap, but take a long time. Sets (the small bulbs) already have a nice head start, but after watching hours of Youtube videos and reading far too many articles, the consensus seemed to be that planting bulbs would result in plants that would bolt quickly, which leads to smaller bulbs and detracts from storage and overall quality of the harvest.

Your onion plants will probably look like this. These are Candy onions

I decided to go with small onion plants, which are just seedlings that have grown to 5-6 inches and are ready to plant. These come in bunches of 30-50, depending on the source. Although they might look dried out or delicate, it is very easy to separate the roots and end up with individual onion plants. If you start them early enough (mid December or so) you can of course just grow these from seed yourself, but I didn't plan ahead so it's plants for me this year!

To separate, gently pull the bunch apart. Then, simply pull out each onion plant, being careful not to damage the roots too much. As I mentioned before, they are a lot tougher than they look, but they will break if you rush it. If you break a few it's no big deal. This was a bunch of 50, so you'll get plenty.

The individual plants ready for the soil!

Step #3: Select and prepare your planter

So now you have your plants ready for soil! Hopefully you already have a number of containers on hand, as well as soil. The question then becomes which container type is best suited to growing onions.

In my case, there are a few considerations. First, space is limited. We have an upstairs porch, so we need to get as much yield from the available space as possible. This means pushing any spacing suggestions right to the limit. In addition, since this is an experiment I don't want to dedicate one of my "premium" containers to onions when there are peppers, tomatoes, eggplants, cucumbers, and many more crops that need a lot of space.

Onions don't grow very deep, but they do need horizontal space to accomodate the bulbs. I decided to use my window boxes, as they are about 36 inches long and hold about 7 gallons of soil, so they are a good size and also allow for a horizontal spread. They can also fit on the porch rails, leaving floor space for the VIP's of the garden. Yes, tomatoes know they are hot shots and have the egos that go with that!

Of course, make sure you use a good potting mix. I have had good luck with any decent quality mix. I look for a light mix (we are upstairs on a porch so weight matters) and one without much, or any, fertilizer, as I like to add my own. Lately I've been using Baccto Pro Mix with good results. It is light, fluffy, and simple. Just make sure to add fertilizer!

Planters filled with mix and ready to go!

Step #4: Space out your plants

In the instructions that came with my candy onions, it was suggested that the onions be spaced 4 inches apart, and 2 inches from the side of the bed (or container in this case). This was the spacing I used for the red onions that I planted a few weeks ago in early April.

Space the plants 4 inches apart 

Also, give them 2 inches from the edge of the bed or container 

More space is always fine, but crowding can result in smaller plants. By following this spacing (with some extra as it turned out) I was able to fit 10 onion plants in each of my 3 foot window boxes. I decided to really push the spacing to the limit and give each plant exactly 4 inches in one of the candy onion boxes, which gave me 16 onions in that container.

All told, I ended up with 2 three foot planters of candy onions, for a total of 26 plants. Earlier in the month I also planted one 3 foot planter of red onions (10 plants) and one 18 inch planter of 5 red onions, for a total of 15. All together, I have 41 onions plants, so this should be an interesting experiment.

Roughly space out the plants

Step #5: Plant the onions (but just the roots!) 

Now it's time for the fun part: planting! The key is to plant shallow. Unlike tomatoes, peppers, and many other garden favorites, onions should be planted very shallow. The instructions that came with these onions direct you to just bury the roots, but leave the white of the onion above ground as much as possible. The reason is that they will apparently form larger bulbs if the bulbs are mostly on top of, rather than buried by, the soil. It makes sense if you've ever seen onions growing, as it looks like they are just sitting on top of the ground.

Make a 1 inch deep hole and just tuck the roots under the soil

Keep most of the white of the onion above the soil

It is going to feel like the plants are very wobbly at first. I was worried about this, as we get a lot of wind on the porch, but you will likely be surprised at how quickly these plants anchor themselves into the soil. Within a few days the red onions I planted, which were much smaller and less green than these candy onions were, were firmly rooted and didn't budge when touched. 

Candy onions just after planting

Once they are planted, water them in as normal, fertilize with a good all-purpose fertilizer, and you should be on the way to successfully growing onions! At least I hope so! We'll see as the season progresses...

In summary: choose the right onion for your latitude, space at least 4 inches, plant shallow, and fertilize!

From everything I've read, onions are an extremely hardy crop. They are tough to kill, withstand frosts, and grow pretty much anywhere. This spring our chives, which were in a small container that was still frozen solid, were growing up through the frozen soil and were a good 6 inches tall before the dirt inside the container finally warmed up and defrosted.

One last great thing about onions: they can (and really should) be planted early, so you can get out in the garden and get some things growing before spring truly arrives. Be sure to check back often for updates on the onions, as well as everything else in the container garden. Also, if you follow Captive Roots on Facebook I will be posting a photo from the garden each day, starting today!

Until next time, happy gardening!